Brass Founding.—The moulds used in the casting of
brass require no new description, the only difference in this
class of work being the manner in which the metal is melted.
As the castings required are much smaller than those we have
recently been describing, the brass is made directly in crucibles
of some,impermeable material, black-lead being the best. The
melting furnaqe is-shewn in Fig. 50, and usually there are several
of these side by side and separately connected with the chimney.
The top of .the furnace is only a little above the floor level, and
in brass foundries it is customary to have the part of the shop
near the furnace entirely reserved for casting purposes; the
casting and moulding shops being entirely divided by a wall
in the best establishments. The principal difficulty in the making
of brass is that of the different fusing points of the two metals
used—Copper and Zinc. Thus, copper melts at 1996° F.,
while the melting point of zinc is as low as 773° F.
The copper is first melted, and the zinc is only introduced a
short time before casting, by means of tongs, pushing it down in
small pieces under the melted copper. It should flare up on
doing this, which is a sign that the heat is quick enough. If it
is left in too long, much of the zinc will be lost by evaporation.*
* See also Appendix I., p. 747.